In many industrial processes, control of film thickness is of critical importance. For example, the manufacture of photographic film requires the generation of a uniform layer of emulsion on a backing. From the point of view of process control, it is advantageous to be able to measure the film thickness during the film generation process rather than measuring the film in a laboratory after the film has been manufactured. If samples are measured off-line, correction of any machinery malfunction cannot be performed until after a considerable volume of defective material has been processed. This leads to waste. For the purposes of the present discussion, the term "film" includes sheets and webs.
Prior art methods for measuring film thickness may be divided into contact and noncontact methods. In one contact method, a micrometer that comes in physical contact with both sides of the film is employed. These methods have the disadvantage of physically deforming the film during the measurement leading to inaccurate measurements and possible damage to the film from pitting or scratching. In addition, the methods are difficult to apply for the on-line measurement of fast moving film webs.
Non-contact methods based on the attenuation of a beam of subatomic particles such as beta particles or gamma rays are also known to the prior art. For example, the attenuation of a beam of electrons by the film is used to determine the film thickness in one prior art method of this type. This methodology has three disadvantages. First, the system must be calibrated for each type of film, since the attenuation depends on the chemical composition and density of the film. Second, the system typically relies on a radioactive source to generate the particle beam. It is generally desirable to limit the use of radioactive material for cost, safety, and psychological reasons. Third, access is normally required to both sides of the film so that the source can be placed on one side and the detector on the other.
Methods for measuring the thickness of films using an optical autocorrelator are also known to prior art. For the purposes of this discussion, an optical autocorrelator is defined to be an interferometer having a variable differential time delay. A Michelson interferometer is an example of such an autocorrelator. For example, U.S. Pat. No. 3,319,515 to Flournoy describes the use of a Michelson interferometer for measuring the thickness of a film. In this system, the film is illuminated with a collimated light beam at an angle with respect to the surface of the film. The front and back surfaces of the film generate reflected light signals. The distance between the two reflecting surfaces is then determined by examining the peaks in the autocorrelation spectrum generated in a Michelson interferometer that receives the reflected light as its input.
The application of this type of autocorrelation technology to the measurement of very thin films has a number of problems. The output of the interferometer is a sinusoidal fringe pattern modulated by an envelope function which exhibits a number of peaks. To accurately measure the thickness of very thin films, the center of each peak must be determined to a high degree of accuracy. In prior art systems, the output of the interferometer is sampled at a rate specified by the Nyquist criterion which results in at least two samples being taken per cycle of the underlying sine wave. In systems requiring fast processing, the resulting number of data points places a significant computational load on the system. To accommodate this load, more expensive computational engines must be used which increases the cost of the measurement system.
The accuracy with which the difference in the peak centers can be determined also depends on the spectral width of the low coherence light source used to illuminate the film. As will be explained in more detail below, broader width sources provide a more accurate determination of the difference. Prior art interferometric film measuring systems use either a white light source or a light emitting diode (LED). While the white light source provides the necessary spectral width, the intensity of light that can be coupled to the film is too low to provide adequate signal to noise ratios in many applications. While an LED source can provide higher power, the spectral line width of the source is too narrow to provide optimum resolution.
Broadly, it is the object of the present invention to provide an improved apparatus and method for measuring the thickness of a thin film.
It is a further object of the present invention to provide a system that does not require contact between the film and the measuring device.
It is a still further object of the present invention to provide a system accurately determine the film thickness without requiring a sampling rate that requires two or more samples per period of the underlying sine wave.
It is yet another object of the present invention to provide a system which achieves the benefits obtainable with wide spectral width light sources while coupling a higher light intensity to the film under measurement than may be coupled using a white light source.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.